One of the challenges faced by biofuel production from microalgae is that, traditionally, the organisms must be harvested and dewatered to reduce the overall processing volume and improve the efficiency of the extraction process. Standard methods available are filtration, centrifugation, sedimentation (or settling), flocculation and membrane filtration. While most of these methods are adapted from existing industrial applications, they suffer from such disadvantages as being time consuming, having significant energy requirements, producing unwanted heat, and being expensive.
Ultrasonic filtration is being investigated for algae harvesting. This approach uses acoustic radiation force to concentrate algae. However, operation of acoustic transducers requires electrical energy input, and the transducers generate significant heat, which needs to be regulated (using additional energy) to avoid unwanted algae lysis.
Commercial centrifuges are the most common instruments being used in algae-based biofuel industries. However, like ultrasonic filtration, operating centrifuges requires electrical energy to operate, and the energy requirement scales proportionally with the volume of operation. Further, this method is labor intensive. In addition, because of the large amount of water involved, centrifugation is cost prohibitive.
Settling, in either holding tanks or plate type settling devices, is an inexpensive though time-consuming process. This method is ill-suited for dewatering algae because of the small size and low specific gravity of algal cells. The length of the settling process for algae poses several difficulties, especially in a production plant where the cells being collected will either be used in a downstream process or for extraction of oil. First, the long processing time decreases plant throughput. Second, it increases the likelihood of contamination that can compromise downstream processing. The long-term storage of the cells associated with settling can lead to decay or reabsorption of the product of interest (e.g., oil, pigments, secondary metabolites, and other co-products) for cell maintenance that can compromise product extraction efficiency and yields.
Chemical flocculation is another harvesting technique, in which specific chemicals are used to flocculate (create aggregates of) the algae. However, this method relies on sedimentation, which is time consuming.
Membrane filtration is also being investigated; however, membrane filters require significant maintenance to avoid clogging.
In recent years, there has been interest in using magnetic means for algae harvesting. However, current approaches require the creation of transgenic algae and/or magnetic beads or nanoparticles, which are time consuming and costly to develop and maintain.
Therefore, there continues to be a need for alternative composition and methods for improved, cost-effective and efficient biomass and biofuel production systems, particularly large-scale systems.